Rod, you don't think his association with Ayers would hurt him on a background check for TS clearance?
No more than I can be called a right wing co-conspirator or sympathizer because I have regular associations with several good friends who are "right wing conservatives." Likewise, there are congressional representatives and senators that are polar opposites politically, ethically and in temperament that sit on the same committees and work on the same subcommittees but that does not make them "guilty by association."
During the course of a national security clearance investigation the nature of the alleged association with Ayers could very well be investigated. The degree of investigation would probably reflect the nature of the background investigation... there is a big difference between a general background investigation and a top level investigation. The most recent comment by "pumpmaster" elevates the level of rigor from another person's original assertion "he would NOT pass the background check" to a much more stringent test "hurt him on a background check for a top secret clearance." If hurt means that an investigation would be more thorough and take longer, yes. However, based upon the open, vetted literature on the "association with Ayers" there is no reason to suspect that Obama would be denied a clearance. On the other hand, it is doubtful that Ayers would certainly receive a top level national security clearance.
Here's a little more that bears on all this "association" logic: "An association fallacy is an inductive formal fallacy of the type hasty generalization or red herring which asserts that qualities of one thing are inherently qualities of another, merely by an irrelevant association. The two types are sometimes referred to as guilt by association and honor by association. Association fallacies are a special case of red herring, and can be based on an appeal to emotion."